Vhf travelling wave loop antenna

ABSTRACT

An improved single antenna means serves two channels of an RDF simultaneously by utilizing each of two receivers (or transmitters) as the antenna termination for the other. The antenna features a novel shape in that it is truncated in width to one-half that of prior antennas.

Hated Sites 1191 [111 ,72,936 Sherman 1 1 Apr. 3, 1973 541 VHF TRAVELLING WAVE LOOP 2,513,736 7 1950 Niutta .343 733 ANTENNA 2,531,432 11 1950 Himmel ..343/736 [75 Inventor: Robert P. Sherman, Dix Hills, N.Y.

[73] Assignee: The United States of America as represented by the'Secretary oi the Primary Examiner-Benjamin A. Borchelt Assistant Examiner-Richard E. Berger Attorney-Richard S. Sciascia et al.

Navy

[22] Filed: Aug. 16, 1971 [57] ABSTRACT [21] Appl. No.: 171,895 An improved single antenna means serves two channels of an RDF simultaneously by utilizing each of two 52 us. 01 .Q ..343/119, 343/736 receive" transmitters) as the antenna terminal [51] Int. Cl ..G01s 3/28 for the other h antenna featul'es a novel Shape in [58] Field of Search"; ..343/1l9, 123, 733, 736 that is truncated width to One-half that Of prior antennas.

[56] References Cited UNITED STATES PATENTS 1 Claim, 2 Drawing Figures 3,064,256 11/1962 Sirons ..343/ll9 X PATEHTEBAPRIB I975 44 46 3O CHAQNEL AMPLITUDE CHAgNEL RECVR' COMP RECVR.

UTILIZATION 52 MEANS FIG. I

. 1 v VHF TRAVELLING WAVE LOOP ANTENNA BACKGROUND OF THE INVENTION This invention relates to antennas and more particularly to directional transmitting and/or receiving VI-IF travelling wave loop antennas.

The form of travelling wave loop antenna conventionally used in amplitude comparison type direction finding systems consists'of an antenna element in the form of a rod having the receiver connected at one end and having the other end of the element terminated in its characteristic impedance by connection to ground through an appropriate resistor. In addition to directionality, such antennas are characterized by broadband capabilities.

Direction finding systems of the amplitude comparison type take advantage of the directional patterns produced by such antennas and typically comprise a plurality of simultaneously operated receiving channels of like frequency, each channel comprising an antenna element and an associated receiver. Comparison means is used to provide an output which is dependent upon relative amplitudes of signals from the channels. This output may be used in various ways, for example as indicative of the direction from which a transmission is being sent. The plurality of antenna elements may consist of groups of two or more arranged to provide the desired azimuthal coverage.

While the term travelling wave loop antenna survives from earlierv configurations it has been found advantageous to use instead of a thin rod an element which is more akin in appearance to a half-rhombic in that the element is folded to slope downwardly in two directions from an apex and isformed-of a thin metal plate or strip of diamond shape. This is done to implement impedance matching. The travelling wave principle is based on the presumption that a wave travels down the antenna in only one direction, with all energy being absorbed at the termination, and nothing being reflected. To implement this, the characteristic impedance of the antenna must be known quite accurately to insure that the antenna can be properly terminated. A thin rod inherently gives rise to impedances which are much higher than would be required for example to match a fifty ohm receiver-impedance. A fifty ohm antenna is much more easily obtained'usinga flat plate arrangement for the conductor. The impedance in this configuration is easily calculable, by treating the antenna as a microstrip transmission line. In this case the characteristic impedance Z, at any point along the line is Z, 377 (h/w) (Equation 1 mentwould suffice, particularly when used in aircraft installations where space and weight considerations are of prime importance.

Additionally, the usual practice of using two antenna elements, one for each of two channels of a comparison system and disposed in 180 relation to one another, results in problems of proper balance and alignment.

SUMMARY OF THE INVENTION With the foregoing in mind, it is a principal object of this invention to provide an improved multiple channel direction finding system wherein a single antenna element serves two channels simultaneously, thereby ef-' apex and to taper outwardly from the ends going toward the apex, but characterized in that the width of the antenna element is truncated at substantially onehalf of the maximum width as it would be in relation to height in accordance with the generally accepted formula stated above as Equation 1. It has been determined byexperiment that in order to achieve a constant impedance it is only necessary to have the width increase linearly with the height until a certain, experimentally determined width is reached. Beyond that point the width may be allowed to remain constant. By so truncating the antenna, the overall size of the antenna is substantially reduced.

The invention may further be said to reside in certain constructions and arrangements of parts whereby the foregoing objects and advantages'are achieved as will be; appreciated from the following description of a preferred embodiment when read in'conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of an antenna embodying the invention shown in conjunction with an amplitude comparison radio direction finder system in block diagrammatic form; and I F IG. 2 is a plan view of the antenna of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT:

In the form of the invention illustrated in the drawings and described hereinafter, there is provided an improved travelling wave loop antenna generally indicated at 10 and shown in FIG. 1 in conjunction with an amplitude comparison direction finding system. The antenna 10 comprises an elongated, flat metallic plate 12 on which is mounted an elongated antenna element 14 formed from a flat plate or strip and folded to have an apex 16. The plate 12 is normally disposed in a horizontal position as illustrated, and the portions 14a,

14b of the element 14 on opposite sides of the apex 16 slope downwardly from the apex in two directions.

The antenna element 14 is supported at opposite ends by insulators 18 and 20 and conductive studs 22, 24. The insulators 18, 20 maintain the element 14 and the suds 22, 24 in spaced relation to the plate 12, while the studs conveniently provide for electrical connection to theends of the antenna element.

The maximum height h of the antenna element 14 is determined according to usual practices, as is the fold angle a. The actual maximum width w/2 of the antenna antenna element, each beam pattern has the gain and element 14 is however a departure from the usual practice which would dictate a maximum width w according to Equation 1 and a diamond shape as indicated by the dot and dash lines 26.

As-may be seen in FIG. 2, the element 14 tapers linearly outwardly in width going from the ends toward the apex. When the width reaches a value of one-half that which is predicted by Equation 1 as a maximum, the element 14 is truncated to maintain a maximum width of w/ 2 through the remainder of the length.

' It has been found by experiment that so truncating the element 14 does not materially reduce the effectiveness of the antenna element, yet a reduction of 2 width to one-half and a somewhat lesser reduction in weight and cost are effected.

Reverting to FIG. 1, the stud 22 is connected as shown by line to provide antenna input to a first receiver 32 serving to receive signals'in a first channel A. The stud 24 is connected as shown by line 40 to provide antenna input to a second receiver 42 serving to receive signals in a second channel B. In this example the A and Bchannels are operated at the same frequency. The receivers 32, 34 serve as amplitude detectors and the outputs thereof on lines 44, 46, respectively, are applied to an amplitude comparator 48. The comparator 48 may be any one of the types normally used in amplitude comparison direction finder systems and feeds its output via line 50 to any desired utilization means 52, e.g., an indicator meter calibrated in direction.

By connecting the receivers 32, 42 to opposite ends of the antenna 10, instead of using a separate antenna for each channel as has been the practice, each receiver appears to the other as the proper termination to the antenna for effective operation, and the antenna gives reciprocal beam patterns for each of the two receiver channels. These beams are directional and, because they emanate or are characteristic of a single behavior of the full antenna aperture. The result is that one antenna 10 serves where two were required before, and additionally, because of physical symmetry, the tracking of the two patterns provided by the antenna 10 will always be better than for two individual antennas.

While the example given has treated antenna 10 as a receiving antenna, it is similarly useful as a transmitting antenna for two transmitters, one connected to each of the ends of the antenna as by studs 22, 24, and each transmitter serving as the'proper antenna termination for the other transmitter.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

5 characterized by improved antenna means comprising:

a flat, horizontal metallic base plate;

an antenna element comprising an elongated metallic strip folded to form an apex and sloping downwardly in two directions from said apex;

insulator means for supporting said element over said base plate and in spaced relation thereto with said apex at a maximum height h above said base plate;

first receiver means for said first receiver channel connected to one end of said antenna element and second receiver channel connected to the other end of said antenna element, whereby each of said receiver means provides the required antenna termination for the other of said receiver means and both receiver channels are operable simultaneously from a single antenna element;

said antenna element tapering outwardly going from each end thereof toward said apex according to the equation v where Z, is av desired impedance, until a width of 

1. An amplitude comparison radio direction finding system of the type comprising antenna means, first and second receiver channels connected to said antenna means, and comparator means operative to provide an output which is a function of the outputs of said first and second receiver channels, said system being characterized by improved antenna means comprising: a flat, horizontal metallic base plate; an antenna element comprising an elongated metallic strip folded to form an apex and sloping downwardly in two directions from said apex; insulator means for supporting said element over said base plate and in spaced relation thereto with said apex at a maximum height h above said base plate; first receiver means for said first receiver channel connected to one end of said antenna element and second receiver channel connected to the other end of said antenna element, whereby each of said receiver means provides the required antenna termination for the other of said receiver means and both receiver channels are operable simultaneously from a single antenna element; said antenna element tapering outwardly going from each end thereof toward said apex according to the equation Zo 377 (h/w), where Zo is a desired impedance, until a width of w(maximum)/2 is reached and then is truncated at a maximum width of w/2 for the remainder of the length of the antenna element. 